This application is a divisional of Ser. No. 09/921,501, filed on Aug. 2, 2001, now U.S. Pat. No. 6,485,655 B1, dated Nov. 26, 2002, and claims benefit thereto. This invention relates to repair of an article that includes a protective internal coating on an internal passage of the article. More particularly, it relates to retention of the internal coating while removing a protective coating on an external surface of the article.
Components of power generation apparatus, for example a turbine engine, operating under relatively high temperature, strenuous environmental conditions generally include surface protection or coatings as a protective barrier between an operating fluid and an article substrate material. One example is a turbine section component of a gas turbine engine operating downstream of a combustor generating products of combustion in an oxidizing atmosphere. Components such as turbine blades and vanes generally are made of a high temperature alloy and are air cooled through at least one internal passage. However, it is common practice to include for additional protection on such component at least an external environmental resistant coating or coating system and frequently an environmental resistant coating on the internal passage.
Examples of high temperature environmental resistant coatings include commercial diffusion aluminide type of coatings, well known and widely used in the gas turbine engine art. Such coatings include Al or an alloy including Al applied to a surface to be protected and heated to diffuse at least a portion of the Al into a substrate of an article. U.S. Pat. No. 3,667,985-Levine et al. describes a form of such an aluminide coating commercially available as Codep aluminide coating. Another type of protective coating widely used commercially with gas turbine engine articles is the M-Cr—Al overlay type of coating in which “M” is at least one of Fe, Co, and Ni, and in which at least one other alloying element, typically Y, has been included. Still another widely used type of aluminide coating used in the gas turbine engine art is the Pt—Al type of coating in which Al is diffused into a Pt layer first deposited on a substrate. Types of such environmentally resistant aluminide coatings, in addition to being used externally, frequently are deposited on a surface of an internal passage of an article, for example on the surface of an internal air cooling passage of a turbine blade or vane.
In some forms of external high temperature environmental resistant coatings, an aluminide coating is used as a bond coat between a substrate and an outer ceramic base coating sometimes referred to as a thermal barrier coating (TBC). Various ceramic materials have been used in the ceramic layer, for example zirconia stabilized with yttria, and magnesia stabilized with yttria.
Periodically, as a result of operation under such strenuous operating conditions including impact on the component surface by fluid borne particles or objects, it is necessary to repair such a component. As practiced in the gas turbine engine art, some repair has required removal of at least a portion of the external coating to enable repair of the coating and/or repair of underlying portions of the component. For example, such repair practice has included at least one of removal and replacement of at least a portion of the external coating, and replacement of a portion of the substrate, for example by welding, brazing, etc. One form of external coating removal, for the aluminide type as well as for the TBC type of coating, has included exposure of the external coating to a medium of a reducing gas including a halide ion or gas, typically of fluoride. Examples of such removal methods are described in U.S. Pat. No. 5,614,054-Reeves et al. (patented Mar. 25, 1997); and U.S. Pat. No. 5,728,227-Reverman (patented Mar. 17, 1998).
Although removal of at least a portion of an external coating can be necessary in the repair of an article, removal of an internal coating on a surface of an internal passage generally is not required in the repair procedure. Exposure of the internal coating to the medium that will remove the internal coating along with the external coating not only is not necessary, but also such removal, requiring subsequent replacement of the internal coating, adds to the cost of repair of the article.